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• avformat_seek_file timestamps not using the correct time base

  19 juin 2021, par Charlie

  I am in the process of creating a memory loader for ffmpeg to add more functionality. I have audio playing and working, but am having an issue with avformat_seek_file timestamps using the wrong format.

  


  avformat.avformat_seek_file(file.context, -1, 0, timestamp, timestamp, 0)


  


  From looking at the docs it says if the stream index is -1 that the time should be based on AV_TIME_BASE. When I load the file through avformat_open_input with a null AVFormatContext and a filename, this works as expected.

  


  However when I create my own AVIOContext and AVFormatContext through avio_alloc_context and avformat_alloc_context respectively, the timestamps are no longer based on AV_TIME_BASE. When testing I received an access violation when I first tried seeking, and upon investigating, it seems that the timestamps are based on actual seconds now. How can I make these custom contexts time based on AV_TIME_BASE ?

  


  The only difference between the two are the custom loading of AVIOContext and AVFormatContext :

  


      data = fileobject.read()

    ld = len(data)

    buf = libavutil.avutil.av_malloc(ld)
    ptr_buf = cast(buf, c_char_p)

    ptr = ctypes.create_string_buffer(ld)
    memmove(ptr, data, ld)

    seeker = libavformat.ffmpeg_seek_func(seek_data)
    reader = libavformat.ffmpeg_read_func(read_data)
    writer = libavformat.ffmpeg_read_func(write_data)

    format = libavformat.avformat.avio_alloc_context(ptr_buf, buf_size, 0,
                                                     ptr_data,
                                                     reader,
                                                     writer,
                                                     seeker
                                                     )

    file.context = libavformat.avformat.avformat_alloc_context()
    file.context.contents.pb = format
    file.context.contents.flags |= AVFMT_FLAG_CUSTOM_IO

    result = avformat.avformat_open_input(byref(file.context),
                                          b"",
                                          None,
                                          None)

    if result != 0:
        raise FFmpegException('avformat_open_input in ffmpeg_open_filename returned an error opening file '
                              + filename.decode("utf8")
                              + ' Error code: ' + str(result))

    result = avformat.avformat_find_stream_info(file.context, None)
    if result < 0:
        raise FFmpegException('Could not find stream info')

    return file



  


  Here is the filename code that does work :

  


      result = avformat.avformat_open_input(byref(file.context),
                                          filename,
                                          None,
                                          None)
    if result != 0:
        raise FFmpegException('avformat_open_input in ffmpeg_open_filename returned an error opening file '
                              + filename.decode("utf8")
                              + ' Error code: ' + str(result))

    result = avformat.avformat_find_stream_info(file.context, None)
    if result < 0:
        raise FFmpegException('Could not find stream info')

    return file


  


  I am new to ffmpeg, but any help fixing this discrepancy is greatly appreciated.

  


• Extracting frames from video with correct DateTimeOriginal exif information

  3 juin 2021, par Matthias

  I have an MP4 file with the creation_time exif information. Now I want to extract each frame from that video and write them into JPG files, however, I also need the correct DateTimeOriginal exif tag in each of these images. So I tried to extract all those frames from the video like this :

  


  ffmpeg -i video.mp4 frames/frame-%05d.jpg


  


  That basically works, but when I now look into a new JPG image it seems there is not DateTimeOriginal exif information. I was hoping that ffmpeg is able to write that tag, based on the position of the extracted frame in the video.

  


  Is that possible ?
  
I am also willing to use another program if ffmpeg is not the right tool for this job.

  


• Can Linux buffer UDP packets to correct the sequence

  6 avril 2021, par QuickPrototype

  Good Day,

  


  I am transmitting MP3 audio using UDP through the Internet. Yes, it's most likely a bad idea for various reasons, however my use case limits me to this setup. To expand further based on comments below, my source device connects over a very high latency link with low and limited throughput as these devices are located in rural/remote areas. My latency is up to 1 seconds (shocking yes). Using TCP will therefore degrade my service due to its two way coms, acks, resends and the likes.

  


  The issue I'm trying to resolve is that on the receiver server (Linux Ubuntu 18.04), I am seeing all the packets but many times slightly out of sequence. I.e. packet 5 comes before packet 4.

  


  Is there a way in Linux to buffer the incoming UDP stream such that consumers (FFMPEG in my case) is able to get stream in order (being a buffer, with an expected delay, that would be linked to buffer size).

  


  Tcpdump example of issue (note : ids are out of sequence) :

  


  16:04:48.648448 IP (tos 0x0, ttl 24, id 25335, offset 0, flags [DF], proto UDP (17), length 108)
    xx.xx.xx.xx.2011 > 172.xx.xx.xx.2011: UDP, length 80
16:04:48.648503 IP (tos 0x0, ttl 24, id 25334, offset 0, flags [DF], proto UDP (17), length 1228)
    xx.xx.xx.xx.2011 > 172.xx.xx.xx.2011: UDP, length 1200
16:04:48.884324 IP (tos 0x0, ttl 24, id 25336, offset 0, flags [DF], proto UDP (17), length 1228)
    xx.xx.xx.xx.2011 > 172.xx.xx.xx.2011: UDP, length 1200
16:04:48.884357 IP (tos 0x0, ttl 24, id 25337, offset 0, flags [DF], proto UDP (17), length 108)
    xx.xx.xx.xx.2011 > 172.xx.xx.xx.2011: UDP, length 80
16:04:49.145213 IP (tos 0x0, ttl 24, id 25339, offset 0, flags [DF], proto UDP (17), length 108)
    xx.xx.xx.xx.2011 > 172.xx.xx.xx.2011: UDP, length 80
16:04:49.145257 IP (tos 0x0, ttl 24, id 25338, offset 0, flags [DF], proto UDP (17), length 1228)
    xx.xx.xx.xx.2011 > 172.xx.xx.xx.2011: UDP, length 1200
16:04:49.406068 IP (tos 0x0, ttl 24, id 25340, offset 0, flags [DF], proto UDP (17), length 1228)
    xx.xx.xx.xx.2011 > 172.xx.xx.xx.2011: UDP, length 1200
16:04:49.406125 IP (tos 0x0, ttl 24, id 25341, offset 0, flags [DF], proto UDP (17), length 108)
    xx.xx.xx.xx.2011 > 172.xx.xx.xx.2011: UDP, length 80
16:04:49.667095 IP (tos 0x0, ttl 24, id 25342, offset 0, flags [DF], proto UDP (17), length 1228)
    xx.xx.xx.xx.2011 > 172.xx.xx.xx.2011: UDP, length 1200
16:04:49.667175 IP (tos 0x0, ttl 24, id 25343, offset 0, flags [DF], proto UDP (17), length 108)
    xx.xx.xx.xx.2011 > 172.xx.xx.xx.2011: UDP, length 80
16:04:49.929139 IP (tos 0x0, ttl 24, id 25344, offset 0, flags [DF], proto UDP (17), length 1228)
    xx.xx.xx.xx.2011 > 172.xx.xx.xx.2011: UDP, length 1200
16:04:49.929200 IP (tos 0x0, ttl 24, id 25345, offset 0, flags [DF], proto UDP (17), length 108)
    xx.xx.xx.xx.2011 > 172.xx.xx.xx.2011: UDP, length 80
16:04:50.164307 IP (tos 0x0, ttl 24, id 25346, offset 0, flags [DF], proto UDP (17), length 1228)
    xx.xx.xx.xx.2011 > 172.xx.xx.xx.2011: UDP, length 1200
16:04:50.164385 IP (tos 0x0, ttl 24, id 25347, offset 0, flags [DF], proto UDP (17), length 108)
    xx.xx.xx.xx.2011 > 172.xx.xx.xx.2011: UDP, length 80
16:04:50.425221 IP (tos 0x0, ttl 24, id 25348, offset 0, flags [DF], proto UDP (17), length 1228)
    xx.xx.xx.xx.2011 > 172.xx.xx.xx.2011: UDP, length 1200
16:04:50.425285 IP (tos 0x0, ttl 24, id 25349, offset 0, flags [DF], proto UDP (17), length 108)
    xx.xx.xx.xx.2011 > 172.xx.xx.xx.2011: UDP, length 80